Recently, the global warming phenomena that result from carbon dioxide have become problems, and reducing the emission of carbon dioxide has become an assignment. Even in automobiles, the reduction of carbon dioxide in the exhaust gases has become an assignment, and accordingly lean-burn engine for lean burning fuel in oxygen-rich atmosphere has been used. Since the usage amount of fuel is reduced by means of this lean-burn engine, it is possible to suppress the emission of carbon dioxide.
And, as a catalyst for converting harmful components in exhaust gases from lean-burn engine, an NOx storage-and-reduction catalyst has been known, NOx storage-and-reduction catalyst on which an NOx storage material being selected from the group consisting of alkali metals, alkaline-earth metals and rare-earth elements is supported along with a noble metal. Using this NOx storage-and-reduction catalyst, and controlling the composition of air-fuel mixture so as to turn it into stoichiometric-rich atmosphere in pulsating manner in the course of lean atmosphere, it is possible effectively to develop the oxidation of HC and CO and the reduction of NOx efficiently, and thereby high converting performance is obtainable.
However, in general NOx storage-and-reduction catalyst, there is such a problem that the NOx storage amount in low-temperature regions is insufficient.
Moreover, in exhaust gases, SOx, which sulfur (S) being included in fuel burns to generate, is included, and then they are oxidized to turn into SO3 by means of catalytic metal in oxygen-rich atmosphere. And, since it is readily turned into sulfurous acid or sulfuric acid by means of water vapor that is likewise included in the exhaust gases, and since these react with NOx storage material to generate sulfites and sulfates, it has become apparent that the NOx storage material has been poisoned to deteriorate. In addition, since porous supporter, such as alumina, has a property of being likely to store SOx, there has been such a problem that the aforementioned sulfur poisoning has been facilitated.
And, when NOx storage material thus turns into sulfites and sulfates, it becomes difficult to store NOx any further, and there has been such a problem that the after-durability converting performance for NOx has declined.
Hence, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2002-11,347, an SOx storage material is set forth, SOx storage material which includes a composite oxide including a rare-earth element and an aluminum oxide, and putting this SOx storage material in place on an upstream side of an NOx storage-and-reduction catalyst is set forth therein.
Moreover, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-113,172, a catalyst for converting exhaust gas is proposed, catalyst in which a barrier layer for suppressing the diffusion of SOx is disposed on an NOx storage-and-reduction catalytic layer as an upper layer. The barrier layer includes an inorganic oxide on which a noble metal and a transition metal are supported. In accordance with this catalyst for converting exhaust gas, since the noble metal oxidized S at the barrier layer in lean atmosphere and then the generated SOx are captured firmly by the transition metal, the SOx are suppressed from diffusing to a lower layer, the NOx storage-and-reduction catalytic layer. And, the noble metal of the barrier layer reduces SOx in stoichiometric-rich atmosphere, then the bond between the transition metal and the SOx are disconnected, and thereby the SOx are released from the barrier layer. Therefore, the SOx storage ability of the barrier layer hardly saturates.
Hence, it is possible to think of putting a catalyst in which only the barrier layer as set forth in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-113,172 is formed in place on an upstream side of an NOx storage-and-reduction catalyst.
In accordance with an exhaust-gas converting apparatus in which a storage catalyst on which an SOx storage material being capable of storing SOx is thus put in place on an upstream side of an NOx storage-and-reduction catalyst, it is possible to suppress the sulfur poisoning of the downstream-side NOx storage-and-reduction catalyst. Moreover, since storing SOx means storing NOx as well, there might be a case where such an advantage is obtainable that the NOx storage amount in low-temperature regions increases.
However, in conventional storage catalyst, since there is a limitation on the supporting amount of SOx storage material so that the storage performance of SOx is not sufficient, there has been such a problem that the sulfur poisoning of NOx storage-and-reduction catalyst, which is put in place on a downstream side, cannot be prevented sufficiently. Moreover, in a case where an SOx storage material is supported in a large amount, there has been such a problem that the SOx storage material reacts with a supporter substrate so that the strength of the supporter substrate has declined considerably.    Patent Literature No. 1: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-113,172; and    Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2002-11,347